Recirculation of exhaust gases has been developed as a method for inhibiting formation of oxides of nitrogen during the combustion process of an internal combustion engine. In general, it is desirable to recirculate exhaust gases at a rate proportional to the rate of engine air flow. To this end, EGR control assemblies have traditionally included an EGR valve having a pintle which is positioned by a vacuum powered diaphragm to provide the desired EGR flow. Such EGR valves are of limited usefulness with unthrottled engines such as Diesel or certain types of direct injected spark ignition engines because such engines typically operate with lower manifold vacuum levels.
U.S. Pat. No. 4,398,524 discloses an EGR assembly including a valve operated by a diaphragm powered by an air pump. U.S. Pat. No. 4,397,289 discloses a conventional unbalanced EGR valve. The EGR valves of the '524 and '289 patents require high forces for valve operation and are limited in flow capacity.
It is an advantage of the present invention that an EGR valve according to this invention does not require high operating forces regardless of the magnitude of the pressure difference across the valve.
It is another advantage of the present invention that an EGR valve according to this invention allows for a large flow area at wide open throttle operation, when the pressure difference across the valve's metering element is small.
It is yet another advantage of the present invention that an EGR valve according to this invention allows very little leakage when the valve is closed.